The present invention relates to a holographic storage material having at least a polymer film and a metallic first layer, wherein the first layer is arranged directly or indirectly on the polymer film.
Holographic storage materials can be such materials to which desired information can be written continuously in the form of a computer-generated hologram. The computer-generated hologram can in this case additionally be continuously readable.
Computer-generated holograms comprise one or more layers of point matrices or point distributions which, in the case of illumination with a preferably coherent light beam, result in a reconstruction of the information which is coded in the hologram. The point distribution can here be calculated as amplitude hologram, phase hologram or as kinoform or from a hybrid form of such types of hologram. In order to produce computer-generated holograms, the latter are first calculated and subsequently written to a storage material using a suitable writing apparatus with the input of energy. The resolution of the resulting point matrix can be within the range of to below 1 μm. It is thus possible to write holograms with a high resolution in a small amount of space, the information of which holograms can be read first by illumination using a light beam and reconstructing the diffraction image. The size of the holograms can in this case be between less than 1 mm2 and several 1 cm2.
The previously described computer-generated holograms can be combined with directly visible information (microscript, microimages).
In addition to computer-generated holograms, the prior art (US 2005/0170259 A1) also discloses embossed holograms. Several embossed holograms can be arranged one on top of the other by way of a multi-layer structure. To this end, it is necessary for the layers to be embossed independently of one another and subsequently be connected to one another. With respect to the respective layers, it is necessary in this security element for them to be sufficiently thick in order to be embossed without the layer being destroyed in the process. As opposed to computer-generated holograms, and owing to the complicated production, embossed holograms cannot be individually designed in series.
A number of writing apparatuses for writing computer-generated holograms are known from the prior art, which write the optical structures of the holograms to planar storage materials. Reference is made here, by way of example, to the documents WO 02/079881, WO 02/079883, WO 02/084404, WO 02/084405 and WO 03/012549.
Likewise known are a number of reading apparatuses which are suitable, by way of illuminating the hologram surface using a light beam and suitable optics, for rendering the reconstruction visible or electronically representable and evaluable by means of recording means. Reference is made here, by way of example, to the documents DE 101 37 832, WO 02/084588 and WO 2005/111913.
Holographic storage materials can be made of a polymer film and a metallic layer whose structure can be influenced in a point-wise manner by way of inputting thermal energy, preferably by means of a focused laser beam.
For the storage material to be effectively writable in the first place, it must have a minimum degree of absorption power. This absorption power is, during the writing process, the triggering factor for the formation of a written point.
The metallic layer has a significant influence on the optical properties, in particular the absorption properties of the storage material. A decisive parameter of the metal layer here is its thickness, since the thickness influences the absorption properties. Consequently, the metallic layer must have a specific thickness in order to ensure sufficient absorption of the radiation which is incident during writing.
In addition to the absorption properties, however, the transmission and reflection also play a role for the properties of the storage material, wherein these further properties are more or less also influenced by the metallic layer. Frequently, a storage material with maximum absorption appears only slightly reflective, grayish and semitransparent. If a more strongly reflective storage material is desired, the metal layer thickness and thus the optical density (OD) can be increased, which, however, results in a noticeable reduction of the absorption and thus a significant increase in the laser power (radiation intensity) required for writing the point distribution. High laser powers, however, are available only to a limited extent or are associated with high costs.
Even if low transmittance is necessary, for example in order to reduce disturbing stray-light influences from the rear of the storage material during reflective reading, the OD needs to be increased. The same is true if the storage material is intended to produce strongly diffractive structures in transmission by way of illumination (e.g. transmission holograms). In this case, the OD likewise needs to be increased. As a result, the abovementioned properties of the storage material can be achieved only to a limited degree or only at high cost.
Therefore, the invention is based on the technical problem of specifying a storage material with improved optical properties.